As is well known, a metal plate, e.g., a stainless steel plate, is generaly widely used as a building material etc. by virtue of its beautiful metallic surface gloss and excellent corrosion resistance. However, even SUS 304 steel believed to have sufficient corrosion resistance when used as the commonest general-purpose steel plate often brings about red rust with time when used in an exposed state for armoring not only in a region where many salt particles come flying from the seek, e.g., near the shoreline, but also in ordinary towns.
The above-described occurrence of rust is mainly due to the "catching rust" accompanying the deposition of floating dust, particularly fine iron powder in the air. This remarkably spoils the image of a high-grade product, i.e., "stainless=free from rust". Thus, an improvement in this respect is one of the big subjects for research in the art even in these days.
A coated stainless steel plate has been developed and commercialized for the purpose of solving the above problems and, at the same time, imparting a decorativeness to attain an attractive appearance as a building material.
Coated stainless steel plate which have been commercialized in these days are classified into those coated with silicone polyester-based resin, fluororesin, acrylic resin, etc. according to the kinds of organic resin paints to be applied on the steel plates. They have been used according to the purposes of use and the performance requirements. Specifically, the steel plates coated with silicone polyester based resin, fluororesin and acrylic resin have been used as a general-purpose opaque resin-coated stainless steel plate, a long-term weather-resistant high-quality coated stainless steel plate and a general-purpose transparent resin-coated stainless steel plate, respectively.
However, these coated stainless steel plates are basically limited to plain-color coating, and the number of colors easily available is limited to several colors called standard colors.
Accordingly, the reason why metal plates, such as stainless steel, having various hues have not been commercialized up to now in spite of their potential needs resides in (1) the restriction derived from the use of a mass production type large-scale high-speed coating line of the roll coat type in the manufacture of a coated stainless steel plate and (2) a disadvantage that in the production of a coated stainless steel plate having a special color other than several kinds of limited colors called standard colors, a mass-production effect is remarkably spoiled due to necessity of manufacture in a small lot, which unfavorably brings about an increase in the production cost.
Further, when a color other than the standard colors is desired, a paint for the special color should be separately purchased from a paint manufacture, which often brings about problems of reproducibility of color in combination with fading of the color caused by ultraviolet rays and needs a long time for the delivery of the paint.
Further, one of the ever-increasing demands in recent years from users for metal plates, such as surface treated stainless steel plates, is to enhance the decorativeness of metal plates such as surface treated stainless steel plates.
The demand for the enhancement of the decorativeness results from the progress of the enhancement of the decorativeness of the interior of the building accompanying an improvement in the level of the civic life in recent years and an ever-increasing demand for individualization, and the conventional coating method of the above-described roll coat type cannot cope with these demands from users unfavorably.
A coated steel plate including the so-called "colored galvanized steel sheet" which uses a plated steel plate as the base plate also brings about the same problem as that of the above-described coated stainless steel plate.
Specifically, although the history of the coated steel plate which uses a plated steel plate as the base plate is longer than that of the coated stainless steel plate and has been more diversified also from the viewpoint of the coating technique, the needs of the users are more and more specialized and complicated.
One of the fields of products having the strongest tendency towards an enhanced decorativeness is that of outer casings of domestic electric appliances. In this field, ultrahigh-quality domestic electric appliances of made-to-order type have come to be manufactured in recent years, which expedites further individualization of such domestic electric appliances.
This means that the production in a small lot and individualized outer casing are necessary. Therefore, manufacturers of coated metal plate are required to develop a manufacturing technique capable of coping with the above-described demands.
Examples of the existing process for manufacturing a colored steel plate include a process for manufacturing a steel plate having a high decorativeness which comprises laminating a printed vinyl chloride resin or fluororesin film on a steel plate as a raw material. However, the printing on this kind of film is basically conducted according to a coloring method suited for mass production, such as the above-described roll coat method, which puts various restrictions on the colored steel plates, so that it is difficult to cope with the individualization encountered in, e.g., the field of ultrahigh quality domestic electric appliances.
Even if a printing method in the sheet form is adopted to cope with the individualization, it is apparent that the cost is unfavorably increased.
Offset printing, silk printing, etc. adopted for cans for beverages made of a tin plate or the so=called "tin-free steel" are very attractive as a method of attaining an enhanced decorativeness. However, in this method, it is necessary to prepare as many expensive printing plates as the number of the colors, which brings about an increase in the cost. Further, during printing, the metal plate should be passed through as many high speed printing machines as the number of the colors and further passed through a large-scale baking oven. Therefore, it is impossible to sufficiently cope with a demand for various kinds of products in small lots.
The conventional coloring technique for the metal plate has been developed for products having a relatively small printing area, such as cans for beverage, and are not always technically suitable for the field where it is necessary to color a building material having a large area, such as wall material.
Further, in special cases, coloring of a stainless steel plate through optical interference of a composite thin film of chromium hydroxide and chromium oxide formed by the so-called "anodic oxidation" including the Inco process which has come to be used mainly for interior ornamentation of, e.g., the wall of an underground street and interior and exterior ornamentation of high-rise buildings in the late several years is attractive from the viewpoint of hue. However, in this method, coloring is basically conducted by immersing a metal plate in a colorant solution, and in order to form a pattern it is necessary to provide very complicated steps, such as a step of masking the metal plate as the raw material for immersion or a step of removing specified colored portions through partial grinding after coloring, so that it is necessary to solve various problems of restrictions and requirements when mass production on a commercial scale is intended.
Further, in order to conduct coloring through optical interference, the color should be controlled by varying the thickness of the thin film. In this case, it is necessary to read the immersion time or a small change in the spontaneous immersion potential, which unfavorably makes it difficult to finely adjust the difference in the color between lots.